78 NAIADACEAE 
33- Potamogeton interruptus Kitaibel. Interrupted Pondweed. (Fig. 174.) 
Potamogeton interruptus Kitaibel in Schultes, OEst. Fl. 
Ed. 2, 328. 1814. 
Potamogeton flabellatus Bab. Man. Bot. Ed. 3, 324. 1851. 
Stems arising from a running rootstock which often 
springs from a small tuber, 2°-4° long, branched, the 
branches spreading like a fan. Leaves linear, obtuse 
or acute, 3/-5’ long, 1//-14’’ wide, 3-5-nerved with 
many transverse veins ; narrow, I-nerved leaves occur 
on some plants and these are acuminate, much like 
those of P. pectinatus ; stipules partially adnate to the 
leaf-blade, the adnate part 14/-1’ long, sometimes with 
narrowly scarious margins, the free part shorter and 
scarious, obtuse ; peduncles 1/-2’ long ; spikes slightly 
interrupted ; fruit broadly and obliquely obovoid, ob- 
tuse at the base, the largest 2’ long and nearly as 
broad, prominently keeled and with rounded lateral 
ridges on the back, the face nearly or quite straight ; 
style facial, erect. 
In ponds and streams, Prince Edward Island to northern 
Indiana and Michigan. Alsoin Europe. August. 
34. Potamogeton Robbinsii Oakes. Robbins’ Pondweed. (Fig. 175.) 
Potamogeton Robbinsii Oakes, Hovey’s Mag. 
7: 180. 1841. 
Stems stout, widely branching, 2°-4° 
long, from running rootstocks sometimes 
1° long. Leaves linear, 3/-5’ long, 2//-3/’ 
wide, acute, finely many-nerved, crowded 
in 2 ranks, minutely serrulate, auriculate 
at the point of attachment with the stipule ; 
stipules with the adnate portion and sheath- 
ing base of the leaf about 4’ long, the free 
part 14’-1’ long, acute, persistent, white, 
membranous, mostly lacerate; peduncles 
1’-3/ long, the inflorescence frequently 
much branched and bearing from 5-20 
peduncles ; spikes interrupted, %’-1’ long, 
flowering under water; fruit obovoid, about 
2’’ broad and 1%4’’ wide, 3-keeled on the 
back, the middle keel sharp, the lateral 
ones rounded, the face arched, the sides 
with a shallow depression which runs into 
the face below the arch; style subapical, 
thick, slightly recurved; apex of the em- 
bryo pointing a little inside the basal end. 
In ponds and lakes, New Brunswick to Oregon, south to New Jersey, Pennsylvania and 
Michigan. The plant is freely propagated by fragments of the stems which throw out rootlets 
from each joint, but this is the rarest of our species to form fruit. Aug.—Sept. 
2m RUPPIATL. Sp. Plo 127. 1753. 
Slender, widely branched aquatics with capillary stems, slender alternate 1-nerved 
leaves tapering to an acuminate apex, and with membranous sheaths. Flowers on a capil- 
lary, spadix-like peduncle, naked, consisting of 2 sessile anthers, each with 2 large sepa- 
rate sacs attached by their backs to the peduncle, having between them several pistillate 
flowers in 2 sets on opposite sides of the rachis, the whole cluster at first enclosed in the 
sheathing base of the leaf. Stigmas sessile, peltate. Fruit a small, obliquely pointed drupe, 
several in each cluster and pedicelled ; embryo oval, the cotyledonary end inflexed, and 
both that and the hypocotyl immersed. [Name in honor of Heinrich Bernhard Rupp, a Ger- 
man botanist. ] 
In the development of the plants the staminate flowers drop off and the peduncle elongates, 
bearing the pistillate flowers in 2 clusters at the end, but after fertilization it coils up and the fruit 
is drawn below the surface of the water. 
